Post by Admin on Sept 1, 2023 9:13:42 GMT
About 1 million years ago, our distant ancestors hunted in small bands and gathered their food with sophisticated stone tools. Then, about 900,000 years ago, something happened: The number of breeding individuals dwindled to only about 1300, according to a new study modeling ancient population sizes. Our ancestors came within a hair’s breadth of extinction, and populations remained that low for the next 100,000 years or more, researchers argue today in Science.
The work, which relies on a new statistical method for estimating ancient population sizes, provides insight into a critical time for our lineage and “fills in a few more pieces of the human evolutionary history puzzle,” says Joshua Akey, an evolutionary geneticist at Princeton University who was not involved with the work.” But others urge caution in adopting its conclusions.
Compared with other living primates, contemporary humans have startingly low genetic diversity. For decades researchers have suspected this arose because our ancestors went through a population bottleneck. To explore further, a pair of researchers from the Chinese Academy of Sciences (CAS), Yi-Hsuan Pan and Haipeng Li, and their colleagues, scrutinized modern human genomes. Scientists know approximately how long it takes for mutations to pile up in our genes, and by looking at variations in genes across different populations, they can estimate how long ago those groups diverged. Researchers can rewind this molecular clock to investigate when major population-shaping events occurred.
Li, a genomicist at CAS’s Key Laboratory of Computational Biology, was interested in populations living 1 million years ago. “That was an interesting time in human evolution,” says Chris Stringer, a paleoanthropologist emeritus at the Natural History Museum in London. Several branches of our closest relatives emerged at that time, one of which gave rise to our lineage.
Li joined forces with Pan, a genomicist at East China Normal University, and together they and their colleagues developed a new statistical approach. To keep computing costs down and reduce errors that come with winding the clock so far back in time, their model only uses a subset of genes—such as those not subjected to forces like positive selection that would change the mutation rate—to estimate population sizes at different points in time.
Using this method, they tabulated when genetic changes had appeared in the previously sequenced genomes of 3154 individuals from 10 modern African populations and 40 modern non-African populations. Population size and history affect the accumulation of these changes, and scientists can analyze them to figure how many people lived at different points in time. Crunching the timelines, Pan and Li found a very steep decline—of roughly 99%—in the breeding population of our ancestors approximately 930,000 years ago. The number of reproducing couples plummeted from at least 100,000 to 1280, they report. (Total population, including children and the elderly, would have been higher.) The low population numbers persisted until about 813,000 years ago, when the number of people began to rise again, the researchers report.
It’s not clear what drove our species to the brink of extinction, but Pan and Li suggest long periods of glaciation, cooling sea surface temperatures, or droughts may have played major parts. “There may be some fine-tuning of the exact time frame and severity,” Akey says, but “the evidence of a severe bottleneck seems pretty compelling.”
However, Janet Kelso, a computational biologist at the Max Planck Institute for Evolutionary Anthropology, is skeptical. She notes that the genetic signal for the bottleneck is strongest only in present-day African populations, and not in people who today live outside Africa, which suggests the bottleneck might have only affected some ancestral populations. The conclusions, “though intriguing, should probably be taken with some caution and explored further,” she says.
Nicholas Ashton, a paleolithic archaeologist at the British Museum who was not involved in the study, agrees. “The hypothesis of a global crash does not fit in with the archaeological and human fossil evidence,” he says. “Questions remain as to what triggered the bottleneck, and what, after 120,000 years, led to expansion.” Still, he adds, “any new way to investigate the past should be welcomed.”
doi: 10.1126/science.adk6174
The work, which relies on a new statistical method for estimating ancient population sizes, provides insight into a critical time for our lineage and “fills in a few more pieces of the human evolutionary history puzzle,” says Joshua Akey, an evolutionary geneticist at Princeton University who was not involved with the work.” But others urge caution in adopting its conclusions.
Compared with other living primates, contemporary humans have startingly low genetic diversity. For decades researchers have suspected this arose because our ancestors went through a population bottleneck. To explore further, a pair of researchers from the Chinese Academy of Sciences (CAS), Yi-Hsuan Pan and Haipeng Li, and their colleagues, scrutinized modern human genomes. Scientists know approximately how long it takes for mutations to pile up in our genes, and by looking at variations in genes across different populations, they can estimate how long ago those groups diverged. Researchers can rewind this molecular clock to investigate when major population-shaping events occurred.
Li, a genomicist at CAS’s Key Laboratory of Computational Biology, was interested in populations living 1 million years ago. “That was an interesting time in human evolution,” says Chris Stringer, a paleoanthropologist emeritus at the Natural History Museum in London. Several branches of our closest relatives emerged at that time, one of which gave rise to our lineage.
Li joined forces with Pan, a genomicist at East China Normal University, and together they and their colleagues developed a new statistical approach. To keep computing costs down and reduce errors that come with winding the clock so far back in time, their model only uses a subset of genes—such as those not subjected to forces like positive selection that would change the mutation rate—to estimate population sizes at different points in time.
Using this method, they tabulated when genetic changes had appeared in the previously sequenced genomes of 3154 individuals from 10 modern African populations and 40 modern non-African populations. Population size and history affect the accumulation of these changes, and scientists can analyze them to figure how many people lived at different points in time. Crunching the timelines, Pan and Li found a very steep decline—of roughly 99%—in the breeding population of our ancestors approximately 930,000 years ago. The number of reproducing couples plummeted from at least 100,000 to 1280, they report. (Total population, including children and the elderly, would have been higher.) The low population numbers persisted until about 813,000 years ago, when the number of people began to rise again, the researchers report.
It’s not clear what drove our species to the brink of extinction, but Pan and Li suggest long periods of glaciation, cooling sea surface temperatures, or droughts may have played major parts. “There may be some fine-tuning of the exact time frame and severity,” Akey says, but “the evidence of a severe bottleneck seems pretty compelling.”
However, Janet Kelso, a computational biologist at the Max Planck Institute for Evolutionary Anthropology, is skeptical. She notes that the genetic signal for the bottleneck is strongest only in present-day African populations, and not in people who today live outside Africa, which suggests the bottleneck might have only affected some ancestral populations. The conclusions, “though intriguing, should probably be taken with some caution and explored further,” she says.
Nicholas Ashton, a paleolithic archaeologist at the British Museum who was not involved in the study, agrees. “The hypothesis of a global crash does not fit in with the archaeological and human fossil evidence,” he says. “Questions remain as to what triggered the bottleneck, and what, after 120,000 years, led to expansion.” Still, he adds, “any new way to investigate the past should be welcomed.”
doi: 10.1126/science.adk6174
